Cellulose nanocrystals reinforced chitosan/titanium dioxide bionanocomposite with enhanced interfacial compatibility: Fabrication, characterization, and application in fresh-cut apple slices.

This research aimed to investigate the feasibility of using a bionanocomposite made of chitosan, CNC, and TiO2 nanoparticles to package freshly sliced apples. At the outset, the effect of varying concentrations of CNC (1, 5, and 10 %) and TiO2 (1, 3, and 5 %) on the mechanical, thermal, and water sensitivity characteristics of the chitosan bionanocomposite was studied. Among different combinations, the bionanocomposite containing 10 % CNC and 3 % TiO2 displayed significant enhancements compared to neat chitosan film. Notably, it exhibited a substantial increase in tensile strength (78.2 %), glass transition temperature (26.7 %), and melting temperature (30.0 %) compared to neat chitosan film. Additionally, it demonstrated reduced WVP (27.8 %), FWS (44.4 %), and SR (50.7 %). These improvements were attributed to the synergistic interactions among chitosan, CNC, and TiO2 nanoparticles through hydrogen and oxygen bonding, corroborated by spectral changes in the material. The photocatalytic degradation of ethylene and microbes by UV-A (intermittent) activated TiO2 contained in the developed bionanocomposite was confirmed by the retention of acceptable quality and radical scavenging activity (70 % retention) of fresh-cut apple slices up to 11 days. The developed bionanocomposite can thus preserve the quality of ethylene-producing horticultural produce.

Relaxation time scales of interfacial water upon fluid to ripple to gel phase transitions of bilayers.

The slow relaxation of interface water (IW) across three primary phases of membranes is relevant to understand the influence of IW on membrane functions at supercooled conditions. To this objective, a total of ∼16.26µs all-atom molecular dynamics simulations of 1,2-dimyristoyl-sn-glycerol-3-phosphocholine lipid membranes are carried out. A supercooling-driven drastic slow-down in heterogeneity time scales of the IW is found at the fluid to the ripple to the gel phase transitions of the membranes. At both fluid-to-ripple-to-gel phase transitions, the IW undergoes two dynamic crossovers in Arrhenius behavior with the highest activation energy at the gel phase due to the highest number of hydrogen bonds. Interestingly, the Stokes-Einstein (SE) relation is conserved for the IW near all three phases of the membranes for the time scales derived from the diffusion exponents and the non-Gaussian parameters. However, the SE relation breaks for the time scale obtained from the self-intermediate scattering functions. The behavioral difference in different time scales is universal and found to be an intrinsic property of glass. The first dynamical transition in the α relaxation time of the IW is associated with an increase in the Gibbs energy of activation of hydrogen bond breaking with locally distorted tetrahedral structures, unlike the bulk water. Thus, our analyses unveil the nature of the relaxation time scales of the IW across membrane phase transitions in comparison with the bulk water. The results will be useful to understand the activities and survival of complex biomembranes under supercooled conditions in the future.

Quantifying dynamical heterogeneity length scales of interface water across model membrane phase transitions.

All-atom molecular dynamics simulations of 1,2-dimyristoyl-sn-glycero-3-phosphocholine lipid membranes reveal a drastic growth in the heterogeneity length scales of interface water (IW) across fluid to ripple to gel phase transitions. It acts as an alternate probe to capture the ripple size of the membrane and follows an activated dynamical scaling with the relaxation time scale solely within the gel phase. The results quantify the mostly unknown correlations between the spatiotemporal scales of the IW and membranes at various phases under physiological and supercooled conditions.

Dehydration induced dynamical heterogeneity and ordering mechanism of lipid bilayers.

Understanding the influence of dehydration on the membrane structure is crucial to control membrane functionality related to domain formation and cell fusion under anhydrobiosis conditions. To this end, we perform all-atom molecular dynamic simulations of 1,2-dimyristoyl-sn-glycero-3-phosphocholine dimyristoylphosphatidylcholine lipid membranes at different hydration levels at 308 K. As dehydration increases, the lipid area per head group decreases with an increase in bilayer thickness and lipid order parameters indicating bilayer ordering. Concurrently, translational and rotational dynamics of interfacial water (IW) molecules near membranes slow down. On the onset of bilayer ordering, the IW molecules exhibit prominent features of dynamical heterogeneity evident from non-Gaussian parameters and one-dimensional van Hove correlation functions. At a fully hydrated state, diffusion constants (D) of the IW follow a scaling relation, D∼τα -1, where the α relaxation time (τα) is obtained from self-intermediate scattering functions. However, upon dehydration, the relation breaks and the D of the IW follows a power law behavior as D∼τα -0.57, showing the signature of glass dynamics. τα and hydrogen bond lifetime calculated from intermittent hydrogen bond auto-correlation functions undergo a similar crossover in association with bilayer ordering on dehydration. The bilayer ordering is accompanied with an increase in fraction of caged lipids spanned over the bilayer surface and a decrease in fraction of mobile lipids due to the non-diffusive dynamics. Our analyses reveal that the microscopic mechanism of lipid ordering by dehydration is governed by dynamical heterogeneity. The fundamental understanding from this study can be applied to complex bio-membranes to trap functionally relevant gel-like domains at room temperature.

A Comparative Study on DMSO-Induced Modulation of the Structural and Dynamical Properties of Model Bilayer Membranes.

Modulating the structures and properties of biomembranes via permeation of small amphiphilic molecules is immensely important, having diverse applications in cell biology, biotechnology, and pharmaceuticals, because their physiochemical and biological interactions lead to new pathways for transdermal drug delivery and administration. In this work, we have elucidated the role of dimethyl sulfoxide (DMSO), broadly used as a penetration-enhancing agent and cryoprotective agent on model lipid membranes, using a combination of fluorescence microscopy and time-resolved fluorescence spectroscopy. Spatially resolved fluorescence lifetime imaging microscopy (FLIM) has been employed to unravel how the fluidity of the DMSO-induced bilayer regulates the structural alteration of the vesicles. Moreover, we have also shown that the dehydration effect of DMSO leads to weakening of the hydrogen bond between lipid headgroups and water molecules and results in faster solvation dynamics as demonstrated by femtosecond time-resolved fluorescence spectroscopy. It has been gleaned that the water dynamics becomes faster because bilayer rigidity decreases in the presence of DMSO, which is also supported by time-resolved rotational anisotropy measurements. The enhanced diffusivity and increased membrane fluidity in the presence of DMSO are further ratified at the single-molecule level through fluorescence correlation spectroscopy (FCS) measurements. Our results indicate that while the presence of DMSO significantly affects the 1,2-dimyristoyl-rac-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-rac-glycero-3-phosphatidylcholine (DPPC) bilayers, it has a weak effect on 1,2-dimyristoyl-sn-glycero-3-phospho-rac-glycerol (DMPG) vesicles, which might explain the preferential interaction of DMSO with the positively charged choline group present in DMPC and DPPC vesicles. The experimental findings have also been further verified with molecular dynamics simulation studies. Moreover, it has been observed that DMSO is likely to have a differential effect on heterogeneous bilayer membranes depending on the structure and composition of their headgroups. Our results illuminate the importance of probing the lipid structure and composition of cellular membranes in determining the effects of cryoprotective agents.

Modulation of Membrane Fluidity to Control Interfacial Water Structure and Dynamics in Saturated and Unsaturated Phospholipid Vesicles.

The structure and dynamics of interfacial water in biological systems regulate the biochemical reactions. But, it is still enigmatic how the behavior of the interfacial water molecule is controlled. Here, we have investigated the effect of membrane fluidity on the structure and dynamics of interfacial water molecules in biologically relevant phopholipid vesicles. This study delineates that modulation of membrane fluidity through interlipid separation and unsaturation not only mitigate membrane rigidity but also disrupt the strong hydrogen bond (H-bond) network around the lipid bilayer interface. As a result, a disorder in H-bonding between water molecules arises several layers beyond the first hydration shell of the polar headgroup, which essentially modifies the interfacial water structure and dynamics. Furthermore, we have also provided evidence of increasing transportation through these modulated membranes, which enhance the membrane mediated isomerization reaction rate.

Dynamic coupling of a hydration layer to a fluid phospholipid membrane: intermittency and multiple time-scale relaxations.

Understanding the coupling of a hydration layer and a lipid membrane is crucial to gaining access to membrane dynamics and understanding its functionality towards various biological processes. To find out how significant the mutual influence of the hydration layer and bilayer dynamics is, a fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) lipid bilayer is simulated atomistically in the presence of the TIP4P/2005 water model at 308 K. Interface water (IW) molecules are classified based on their continuous physical proximity or ability to form hydrogen bonds with different moieties of lipid heads. A gradient in retardation of translational mean square displacements is found to operate coherently for both IW and lipid components across the bilayer normal. Deviations from Gaussianity in van Hove correlation functions increase for the lipids and decrease for the IW from the tails to the heads. The IW molecules exhibit Fickian but intermittent dynamics due to coupled vibrations in the local cage formed by the hydrogen bonds with the lipid heads followed by decoupled translational jumps. Importantly, the differences in regional dynamics of lipid heads are clearly reflected in the dynamics of spatially resolved IW molecules physically close to the lipid heads, but not to the dynamics of the hydrogen bonded IW molecules far from the lipid heads. These analyses imply that spatially resolved interface water dynamics can act as a sensitive reflector of regional membrane dynamics occurring at sub ps to hundreds of ps time-scales for several important biological functions at physiological temperature in the future.